Japanese Patent Application Laid-Open (JP-A) No. 2004-149074 discloses a vehicle impact absorbing member of the above-described type. The absorbing member has a hollow double-wall structure produced by blow molding of a thermoplastic resin and has a hollow portion. Recessed ribs are formed from front and rear surface walls, and end portions thereof are boded to one another to be integral, thereby to improve impact absorbing property. In addition, the official gazette of Japanese Patent No. 3313999 describes a vehicle impact absorbing member formed from a polypropylene resin having a bend elastic modulus of 5000 to 2500 kg/cm2.
JP-A-2000-193008 describes an impact absorbing member, in which slits or notch shapes are formed as breaking triggers in a cylindrical impact energy absorbing member, thereby to cause stable self-breaking. Further, JP-A-2000-177518 discloses a blow-molded hollow-bag shaped in-vehicle lateral side collision dedicated pad, in which the entirety of the pad is caused to be broken by a stress concentration portion to thereby absorb collision energy.
In a vehicle impact absorbing member formed from a blow-molded hollow double-wall structure such as described in JP-A-2004-149074, the impact absorbing member, especially, wall surfaces of the recessed ribs are broken in such a manner as to be crushed during the absorption of energy caused by impact. In conjunction with crushing of the impact absorbing member, air present inside the hollow portion of the impact absorbing member is compressed thereby to increase internal pressure to the extent of imposing influence on the impact absorption property. Such an influence, generally, causes fluctuations in load value, consequently making it difficult to regulate the impact absorption performance.
As in the case of the vehicle impact absorbing member described in the official gazette of Japanese Patent No. 3313999, the configuration formed from the polypropylene resin having the bend elastic modulus of 5000 to 25000 kg/cm2 is influenced by the air present inside the hollow portion. In addition, in such a configuration, the stresses in the event of impact absorption is varied by temperature variations of fresh air in the range of from 60° C. to −15° C. Consequently, similarly as above, fluctuation or reduction occurs in load value, thereby causing variations in the impact absorption performance. More specifically, with respect to a reference set to the load stress in the ambient temperature at a 50% compressive strain of the impact absorbing member formed from the polypropylene resin, a stress variation rate is 21% at the outside air temperature of −15° C., and the stress variation rate at the outside air temperature of 60° C. is −34° C.
Under these circumstances, there are placed demands on vehicle impact absorbing members of the type such as described above for removal of factors of influencing the impact absorption performance, setting of the impact absorption performance by using control means (which, in the present case, corresponds to, for example, the shapes of or the number of the recessed ribs, or the wall thicknesses of walls constituting the impact absorbing member), and improvement of temperature dependency of the impact absorption performance.